Rearrangements of 1,2,3,4,5‐, 1,2,4,4,5‐, 1,3,4,4,5‐ and 1,2,3,4,4‐pentamethylcyclopentenyl ions and of 1,3,4,4,5,5‐ and 1,2,3,4,4,5‐hexamethylcyclopentenyl ions. stability of pentamethylcyclopentenyl ions

Abstract

AbstractA detailed investigation has been made of the reversible rearrangement of the 1,2,3,4,5‐pentamethylcyclopentenyl ion (1) to the 1,2,3,4,4‐pentamethylcyclopentenyl ion (3). In 5: 1 FSO3H‐SbF5 this reaction proceeds entirely intramolecularly by 1,2‐hydrogen and methyl shifts via three allylic and four non‐conjugated tertiary pentamethylcyclopentenyl ions as intermediates. Two of the intermediate allylic ions have been prepared by cyclization of the 2,4,6‐trimethylheptadienyl ion and their rearrangements have been further investigated.Rate constants of the various rearrangements are reported and a detailed free‐energy profile of these reactions is presented.The 1,3,4,4,5‐pentamethylcyclopentenyl ion (4) is 4.7, and the 1,2,4,4,5‐isomer (5) 10.0 kcal · mole−1 less stable (in terms of ΔF at 50°) than 1; these large differences are partly ascribed to the crowding of the three methyl groups on C4 and C5 in the former two ions.The cation 1 occurs as cis and trans isomers, which are formed in a 1: 1 ratio upon protonation of the diene, and in a 2.3: 1 ratio upon rearrangement of 5 and 4. Their equilibrium ratio is 0.37: 1, which may indicate a non‐planar structure for this ion.Our re‐examination of the cyclization of the 2,4,6‐trimethylheptadienyl ion in FSO3H‐SbF5 gave results considerably different from those reported by previous workers.In the 1,2,3,4,4,5‐hexamethylcyclopentenyl ion (2) degenerate equilibrations through alternating hydrogen and methyl shifts cause line broadening in the PMR spectrum above 120°. These shifts are faster than the corresponding shifts of hydrogen in 1 and of methyl in 3, respectively, which is attributed to a steric acceleration due to the crowding of the three methyl groups on C4 and C5 in 2. Similar steric effects are held responsible for the fact that the rearrangement of 1,3,4,4,5,5‐hexamethylcyclopentenyl ion to 2 is much faster than that of 4 to 1 and 3, and that in 5 the methyl shift is almost as fast as the hydrogen shift.